Add support for globbing one level down

Initially I limited it to only the specified folder, because of performance problems we experienced in 0.9.x.

There has come up a lot of questions about our globbing. Especially about the use-case of having eg. images in separate folders `app/images/ui/` `app/images/content`.

This proposal adds the ability to glob one level down. Which means it would match every image in `app/images` and `app/images/ui`, but not `app/images/ui/anotherdir`.

What do you think? Can you also look through the PR and see if it's ok.

-

@sleeper @btford @passy @addyosmani